It is proposed to perform an in vitro and animal in vivo investigation of 188Re labeled to several antibodies by several direct and indirect methods in a manner similar to that just completed for 99mTc by the applicant. Stability of the label will be assessed in vitro by saline and serum incubations and by a novel cysteine challenge assay. Biodistribution studies in normal and in tumor-bearing mice will provide information on the in vivo fate of each suitable label. The applicant anticipates that this research will show definitively the superiority of one labeling method (e.g. direct vs. indirect) in mice and will, therefore, provide an objective approach towards the eventual investigation of these reagents for therapy of cancer patients.